To provide a sub muffler for an exhaust system of an automobile capable of increasing an output by increasing exhaust speed of exhaust gas discharged from an engine.

This sub muffler 1 disposed so as to decrease pressure of the exhaust gas and discharge it comprises a pipe-like case body 2 having both opening ends, an insulating material 5 that has an insert hole 5a in its center and is inserted into the case body 2, a hollow member 3 engaged with the insert hole 5a so as to tightly adhere to its inner surface, a pressure reducing member 4 that is disposed inside the hollow member 3 and has many through- holes in the outer peripheral surface, a voltex flow forming member 6 inserted into the inner surface of the pressure reducing member 4, caps 8 that are disposed at both ends of the case body 2 and have a connecting pipe 7, and a predetermined space 10 disposed between the inner surface of the hollow member 3 and the outer surface of the pressure reducing member 4. The exhaust gas discharged from the engine is changed from slow flow to excess flow through the voltex flow forming member 6, and discharged to the atmosphere at a high speed to decrease pressure in the pipe and increase the output.